1. Field of the Invention
The present invention relates to a charged-particle beam instrument, such as a transmission electron microscope (TEM), for observing or analyzing a specimen by irradiating the specimen accommodated in a specimen chamber with an electron beam. More specifically, the present invention relates to a charged-particle beam instrument permitting one to observe the process of a reaction of a specimen with a gas when the specimen is placed in an ambient of the gas.
2. Description of Related Art
A charged-particle beam instrument, such as a transmission electron microscope (TEM), is sometimes required to make in situ dynamic observation of a specimen placed in an ambient of a gas. For example, in the field of research on catalysts, dynamic observation of a process in which particles of a catalyst react with a gas and undergo changes plays a great role in improving the catalyst. Furthermore, it is expected that making a dynamic observation to understand how a material is varied by a gas will be helpful to research on corrosion of the material, for example, due to environmental pollution gases and to improvements of the material. Furthermore, in observations of biological specimens, if a specimen is placed in a vacuum, the specimen dries. Therefore, observing a specimen after it has been placed in a given ambient gas is an important technique.
Where a specimen is placed in an ambient of a gas and observed dynamically with an electron microscope by the prior art technique, the used electron microscope is a specially designed microscope in which the specimen chamber itself of the microscope is a chamber containing a gaseous ambient as described in JP-A-2003-187735.
FIG. 7 is a vertical cross section of the electron optical column of a conventional transmission electron microscope (TEM) that is one kind of charged-particle beam instrument.
As shown in FIG. 7, the electron optical column indicated by reference numeral 101 is made of a substantially cylindrical outer cylinder 102. If necessary, the inside of the outer cylinder 102 is partitioned into discrete zones by partition walls. Near the center axis of the electron optical column 101, the portions constituting a flight path 105 of an electron beam 103 form an axially continuous space that is evacuated to a vacuum.
An electron source 104 is mounted inside the electron optical column 101 and emits the electron beam 103. A condenser lens 106, an objective lens 107, and other electron lenses for diffusing and converging the electron beam 103 utilizing magnetic or electric fields are incorporated in the electron optical column 101. Furthermore, an aperture stop 108 for removing unwanted electrons and shaping the electron beam 103 is mounted in the electron optical column 101. An aperture stop driver 109 for adjusting the position of the aperture stop 108 is also mounted in the column. A specimen holder 111 that is a stage on which a specimen 110 to be observed and analyzed is placed is also mounted in the column. A specimen holder driver 112 for adjusting the position of the specimen holder 111 is also mounted in the column.
The specimen holder 111 has a front-end portion placed within the specimen chamber 113 that is located within the electron optical column 101. The specimen 110 is introduced into the specimen chamber 113, where observation and analysis are performed. Normally, the specimen chamber 113 is formed inside the objective lens 107.
FIG. 8 is a vertical cross section showing the configuration of the objective lens 107 of the conventional transmission electron microscope (TEM). The objective lens 107 is made up of a yoke 114, an excitation coil 115, and a polepiece assembly 116 inside the outer cylinder 102 as shown in FIG. 8. The yoke 114 is made of a material having a high magnetic permeability. The yoke is fabricated by closing the upper and lower ends of a double cylinder. An upper portion of the inner cylinder has been cut out. A beam passage hole 117 is formed in the center of the yoke 114 and extends in the up-and-down direction. The electron beam 103 passes through the passage hole 117. The excitation coil 115 is received between the two cylinders of the yoke 114. The coil 115 is fabricated by cylindrically winding many turns of a metal wire, such as a copper wire, coated with an insulator.
The polepiece assembly 116 of the objective lens is made of a material having a high magnetic permeability, and is shaped substantially cylindrically. The polepiece assembly 116 is made up of a top polepiece 118, a bottom polepiece 119, and a spacer 120 made of a nonmagnetic material, such as a copper alloy. Each of the top polepiece 118 and bottom polepiece 119 has a protrusive portion obtained by trimming away a conic top portion. The protrusive portions of the top polepiece 118 and the bottom polepiece 119 are opposite to each other and spaced apart a given distance. The top polepiece 118 and bottom polepiece 119 have beam passage holes 121 and 122, respectively, permitting passage of the electron beam 103. The holes 121 and 122 extend in the up-and-down direction. The spacer 120 connects the top polepiece 118 and the bottom polepiece 119 while maintaining the space between them. The polepiece assembly 116 of the objective lens is mounted in an upper cutout portion of the inner cylinder of the yoke 114.
In the transmission electron microscope, a specimen 110 is inserted in the specimen chamber 113 and observed or analyzed, the chamber 113 being located between the opposite top polepiece 118 and bottom polepiece 119. The position at which the specimen 110 is installed can be termed the specimen observation position 123. Inside the yoke 114, a specimen stage 124 made of a nonmagnetic material is mounted at a side of the specimen chamber 113. Plural connection flanges 125 radially extend through the outer cylinder 102 and yoke 114 on the side of the outer periphery of the specimen stage 124. The specimen holder driver 112 and pipes (not shown) for vacuum pumping are held to the connection flanges 125.
The specimen stage 124 is provided with a hole 126 extending through it. The hole 126 is formed to extend toward the specimen observation position 123. The specimen holder 111 can be introduced via the hole 126 to bring the specimen 110 into the observation position 123 or the observation position 123 can be evacuated to a vacuum. If necessary, a through-hole 127 for the specimen holder 111 is formed in a side portion of the spacer 120.
Required connected portions of the components of the objective lens 107 are hermetically sealed with O-rings 128, 129, 130, 131, 132, and 133. The space around the specimen observation position 123 is maintained as a vacuum. The space including the observation position 123 and maintained as a vacuum is referred to as the specimen chamber 113.
In the objective lens 107, the specimen chamber 113 is evacuated to a vacuum via the through-hole 126 in the specimen stage 124. The spaces above and below the polepiece assembly 116 of the objective lens are also evacuated to a vacuum with other vacuum pumping piping (not shown). In this case, all vacuum pipes branch off from the same main pipe. The beam passage holes 121 and 122 formed in the top polepiece 118 and bottom polepiece 119 are sufficiently large and so the degree of vacuum is substantially uniform from location to location.
FIG. 9 is a vertical cross section showing the structures of main portions of an objective lens 107′ incorporated in a transmission electron microscope (TEM) having a conventional specimen chamber containing an ambient of a gas. This objective lens 107′ has a top polepiece 118 and a bottom polepiece 119 provided with beam passage holes 121 and 122, respectively, as shown in FIG. 9. Aperture stops 135, 136, 137, and 138 may be mounted above and below (locations closer to and remote from the specimen 110) the beam passage holes 121 and 122. Each aperture stop is a disk made of a nonmagnetic metal centrally provided with a small orifice. The aperture stops 135 and 136 closer to the specimen 110 are referred to as the first upper aperture stop and the first lower aperture stop, respectively. The aperture stops 137 and 138 further from the specimen are referred to as the second upper aperture stop and the second lower aperture stop, respectively. Each aperture stop is mounted to the top polepiece 118 and bottom polepiece 119 such that the connections are made as hermetic as possible. For example, each aperture stop is bonded with a conductive adhesive.
In this case, the space formed between the first upper aperture stop 135 and the second upper aperture stop 137 is referred to as the upper intermediate chamber 139. The space formed between the first lower aperture stop 136 and the second lower aperture stop 138 is referred to as the lower intermediate chamber 140. The upper intermediate chamber 139 is in communication with the vacuum space located above the polepiece assembly 116 of the objective lens and with the specimen chamber 113 only via small aperture holes 141 formed in the second upper stop 137 and first upper aperture stop 135. Similarly, the lower intermediate chamber 140 is in communication with the vacuum space located below the polepiece assembly 116 of the objective lens and with the specimen chamber 113 only via small aperture holes 141 formed in the second lower aperture stop 138 and first lower aperture stop 136.
An upper differential pumping tube 142 and a lower differential pumping tube 143 extend through the outer cylinder 102, yoke 114, specimen stage 124, and spacer 120 and are connected with the upper intermediate chamber 139 and lower intermediate chamber 140, respectively. Required portions are kept hermetic with O-rings 144.
In the objective lens 107′, the pumping system for evacuating the specimen chamber 113 is separate from the upper differential pumping tube 142 and lower differential pumping tube 143 which evacuate the upper intermediate chamber 139 and lower intermediate chamber 140, respectively. The pumping system for evacuating the specimen chamber 113 is separate from pumping systems for evacuating the spaces located above and below the polepiece assembly 116 of the objective lens. The aperture holes 141 are present among the specimen chamber 113, upper intermediate chamber 139, lower intermediate chamber 140, and the spaces located above and below the polepiece assembly 116 of the objective lens. Because admission and venting of gas are limited, pressure differences can be created among the spaces. Accordingly, in the objective lens 107′, a trace amount of arbitrary gas can be introduced into the specimen chamber 113 through the specimen stage 124 such that the degrees of vacuum in the spaces located above and below the polepiece assembly 116 of the objective lens are hardly affected. The specimen 110 can be observed and analyzed under arbitrary gaseous environments.
The above-described charged-particle beam instrument has the problem that it is difficult to exchange the first upper aperture stop 135 and first lower aperture stop 136. That is, if the diameter of the aperture hole 141 is varied or if contamination has occurred due to the charged-particle beam, it is necessary to exchange the first upper aperture stop 135 and first lower aperture stop 136. However, the first upper aperture stop 135 and first lower aperture stop 136 are adhesively bonded to the sides of the top polepiece 118 and bottom polepiece 119 facing the specimen chamber 113. Therefore, it is impossible to exchange them unless the polepiece assembly 116 of the objective lens is taken out, for example, by disassembling the electron optical column 101. Consequently, it is difficult to exchange the first aperture stops. In addition, there is the problem that it is impossible to adjust the positions of the first upper aperture stop 135 and first lower aperture stop 136 or to vary the diameter during microscopic examination.
Additionally, the charged-particle beam instrument has the problem that it is difficult to maintain the accuracy of the positions at which the first upper aperture stop 135 and first lower aperture stop 136 are mounted. In particular, the first aperture stops 135 and 136 are held in position after the polepiece assembly 116 of the objective lens has been taken out of the electron optical column 101. Hence, it is difficult that their positions within the electron optical column 101 are previously checked and that the first aperture stops are placed in position.
Moreover, in this charged-particle beam instrument, the first aperture stops 135 and 136 are kept on the passage of the electron beam 103. Therefore, there is the problem that it is difficult to use the instrument if a method not using the aperture stops 135 and 136 is employed. That is, when it is necessary to remove the first aperture stops 135 and 136, the polepiece assembly 116 of the objective lens must be taken out, for example, by disassembling the electron optical column 101.